In economically developed countries, cardiovascular disease represents a major cause of mortality. In particular, abnormal coagulation and inappropriate thrombus formation within blood vessels precipitates many acute cardiovascular disease states. While it has long been recognized that a variety of plasma proteins such as fibrinogen, serine proteases, and cellular receptors are involved in hemostasis, it is abnormal regulation that has emerged as important contributing factors to cardiovascular disease.
Thrombin can be considered the key or principal regulatory enzyme in the coagulation cascade; it serves a pluralistic role as both a positive and negative feedback regulator in normal hemostasis. However, in some pathologic conditions, the positive feedback regulation is amplified through catalytic activation of cofactors required for thrombin generation. Such cofactors include factor Xa, a serine protease which occupies a pivotal position in the coagulation cascade. Factor X is the zymogen of factor Xa. Factor X can be activated either the intrinsic or extrinsic pathways of the coagulation system. Initiation of coagulation by either pathway in response to vascular injury activates factor X to factor Xa. Factor Xa and its cofactor, factor Va, combine on a phospholipid membrane to form the “prothombinase” complex, which activates prothrombin to thrombin. Thrombin cleaves fibrinogen to fibrin, activates platelets, and converts factor XIII to XIIIa which is the principal enzyme involved in thrombus generation, growth, and stabilization. Accordingly, the location of the prothrombinase complex at the convergence of both the intrinsic and extrinsic coagulation pathways suggests that inhibition of factor Xa, and hence thrombin generation, may be a viable approach to limiting the procoagulant activity of thrombin.
Evidence exists for the role of factor Xa inhibitors as anticoagulants. Antistasin, a potent inhibitor of blood coagulation factor Xa from the Mexican leech, Haementeria officinalis, displays antithrombotic activity in various models of arterial and venous thrombosis (Lapatto et al., Embo. J, 1997:5151-5161). Other protein or polypeptide factor Xa inhibitors include recombinant tick anticoagulant peptide (rTAP), which is known to accelerate the recombinant tissue plasminogen activator mediated clot lysis and prevent acute reocclusion in the dog, hence indicating factor Xa inhibitors may be useful as an adjunct to thrombolytic therapy (Mellott et al., Fibrinolysis, 1993: 195-202). Furthermore, in a canine coronary artery electrolytic lesion model, rTAP was demonstrated to reduce thrombus mass and time to occlusion in the absence of dramatic hemodynamic or hemostatic changes indicating the primary role for factor Xa in the process of arterial thrombosis (Lynch et al., Thromb. Haemostasis, 1995:640-645; Schaffer et al., Circulation, 1991: 1741-1748). On the venous side, rTAP was also demonstrated to reduce fibrin deposition in a rabbit model of venous thrombosis while having little affect on systemic hemostatic parameters (Fioravanti et al., Thromb. Res., 1993: 317-324). In addition to these relatively high molecular weight proteins that are not suitable as oral antithrombotic agents, there also exist examples of low molecular weight factor Xa inhibitors. In particular DX9065a, a low molecular weight synthetic factor Xa inhibitor, has also shown antithrombotic potential in various experimental thrombosis rat models. In both arteriovenous shunt and venous stasis models, inhibition of thrombus formation was achieved at doses that had little effect on APTT, indicating that DX9065a is effective in preventing thrombosis and hence has therapeutic antithrombotic potential (Wong et al., Thromb. Res., 1996: 117-126).
Recently, it has been appreciated that factor Xa inhibition may provide sustained antithrombotic protection. Specifically, several animal studies show that inhibition of short term exposure to factor Xa produces a sustained antithrombotic. (Leadley, Curr. Top. Med. Chem., 2001: v. 1, 151-159.) Finally, the article by Leadley observes that factor Xa inhibition potentially provides a large therapeutical window between antithrombotic efficacy and bleeding tendency. Consequently, there may exist a range in which factor Xa inhibition is achieved without an concurrent increase in a patients susceptibility to bleeding.
The majority of factor Xa inhibitors known to date have been summarized in two reviews (Edmunds et al., Annual Reports in Medicinal Chemistry, 1996:51 and Kunitada and Nagahara, Curr. Pharm. Des., 1996:531-542). However, it is readily apparent that there still exists a need for more effective agents that regulate factor Xa proteolytic activity.